The present disclosure generally relates to acquisition of direct-sequence spread-spectrum (DSSS) signals.
As discussed in Chege, “Acquisition of Direct-Sequence Spread-Spectrum Signals,” 2011, DSSS is a spread-spectrum signaling technique in which a data sequence is used to modulate a wideband code. The data sequence is a fast pseudo-randomly generated sequence, with the result that the narrowband data sequence is transformed into a wideband noise-like signal. The resulting wideband signal then undergoes a second stage of modulation, where phase-shift keying is used. Synchronization of the spreading code is of utmost importance in any spread-spectrum system. The proper operation of the system depends on how well synchronization is done. A solution to the synchronization problem consists of two parts, referred to as acquisition and tracking. Acquisition may be regarded as coarse synchronization and, being the first step in the synchronization procedure, must work very efficiently, after which tracking can be performed and synchronization ultimately achieved.
A DSSS system uses a noise-like spreading code referred to as a pseudo-noise (PN) sequence. As further discussed in Chege, a PN sequence is a periodic binary sequence with an autocorrelation that resembles, over a period, the autocorrelation of a random binary sequence. Its autocorrelation also roughly resembles the autocorrelation of bandlimited white noise. Ideally, one would want to use a truly random binary sequence. However, for the data to be recovered, the receiver needs to know the code that was used for spreading. This knowledge is unavailable in a non-deterministic process like a random binary sequence, hence the use of PN sequences, which are deterministic.
As further discussed in Chege, at the spread-spectrum receiver, the corresponding chips in the spreading sequence must precisely or nearly coincide. Any misalignment causes the signal amplitude at the demodulator to fall in accordance with the autocorrelation function, leading to signal degradation. Synchronization includes acquisition, wherein the phase, or delay in time, of the receiver-generated sequence is brought within phase of the received sequence.
Longer spreading codes have the advantage of greater security and the possibility of more low cross correlation codes operating simultaneously. However longer codes also require more time to acquire. This procedure provides a quick way to identify and determine the sequence timing of a spreading code. In prior art systems, individual code blocks check the correlation of a single alignment by reaching an integrated data signal power threshold within a given time period. Only a small subset of the total number of possible alignments is checked at a time, sequentially testing subsets until the alignment is found. If the alignment is changing with time (as is the case with a Doppler shifted signal), it is possible that alignment will switch to a previously tested location, and thus be missed. If alignment is not found after testing all possible alignments, the process is repeated.
FIG. 1 illustrates a prior art DSSS system 100. As shown in the figure, DSSS system 100 includes a transmitter 102 and a receiver 104.
In operation, at transmitter 102, data 106 is exclusively or'ed (XORed) with a spreading sequence PN 108 to generate a DSSS signal 110. Transmitter 102 transmits DSSS signal 110 to receiver 104, wherein along the transmission rout, DSSS signal 110 encounters noise 112 so as to create a received signal 114.
At receiver 104, a corresponding spreading sequence PN 116 is XORed to received signal 114 to extract the data for the channel that was “spread” by transmitter 102 with the same spreading sequence. However this only works when the receiver spread code is in perfect alignment with identical spread code of the transmitter. Before a receiver can extract the channel of interest, it must first determine the timing of the beginning of the transmitted spread code. This is usually done by checking each possible alignment, i.e., checking each phase delay, until the correct start alignment produces the anticipated data. It is assumed the correct alignment will produce the most transmitted power at frequencies less than the “chip” rate of the transmission. Time at each possible alignment is required to integrate this power by integrator 118 to see if it surpasses an arbitrary threshold. Eventually, after alignment is acquired, the decoded data 120 is passed to downstream circuitry for further processing.
FIG. 2 illustrates a graph 200 of integrated power over time of the prior art DSSS system of FIG. 1. As shown in the figure, graph 200 has a y-axis 202 in units of accumulated power, an x-axis 204 in units of time, a power threshold Pth 206, a time threshold tth 208, a function 210 and a function 212.
As mentioned above, in DSSS system 100, receiver 104 must check each possible alignment for acquisition. When summing the correlated values of the received signal 114 with PN 116 at one phase, the accumulated power will eventually increase, even if there is no alignment. While the accumulated power eventually increases over time, if the rise in power takes too long, then alignment is likely not present. Accordingly, when designing DSSS system 100, the power threshold Pth 206 and time threshold tth 208 are set to weed out unlikely prospects for alignment. For example, function 210 eventually reaches acceptable power threshold Pth 206, meaning that over time, sufficient pits of PN 116 correlate with corresponding pits of the received signal 114, which has been offset by a predetermined phase, so as to generally increase the accumulated power. However, the rate of the rise of accumulated power of function 210 is insufficient to meet Pth 206 prior to time threshold tth 208. Accordingly, a new phase will be used and the process will repeat. This will continue until an phase will provide an accumulated power function that reaches acceptable power threshold Pth 206 prior to time threshold tth 208. This is represented by function 212.
The problem with DSSS system 100 is that many alignments may need to be tested, each of which until tth is reached, which wastes much time and processing resources.
There exists a need for a system and method for quickly and efficiently identifying the transmitted spread code sequence offset alignment and offset time dependence, even in the presence of a Doppler shift.